9% is Ideal forward of Center
here is the most accurate way to determine forward of center...

Get a long finishing nail and nail it into a wall just enough to get it snug without falling out. Now with your finished arrow find the balancing point on the arrow and mark that spot on the arrow.

Measure from the throat of the nock to the end of the shaft ( do not measure the point or insert) this is the cut length of your arrow.

Now, divide this measurement by 2 this will give your center of the arrow measurement.

Mark the center of the arrow (from the throat of the nock to the center).

Measure the distance between the balancing point to the center.

Multiply this measurement by 100 then divide that multiple by the arrows cut length.

This will give you your FOC%